1. Field of the Invention
This invention relates to a liquid crystal display device, specifically to a liquid crystal display device in which the alignment direction of liquid crystal molecules is controlled by an electric field between electrodes on the same substrate.
2. Description of the Related Art
As one way of achieving a wide viewing angle for a liquid crystal display device, a method has been developed to realize a light switching function by rotating the liquid crystal molecules in a plane parallel to the substrate with a lateral electric field generated between the electrodes on the same substrate. In-Plane Switching (hereafter referred to as IPS) method and Fringe-Field Switching (hereafter referred to as FFS) method are examples of such a technology.
A conventional liquid crystal display device using the FFS method will be explained referring to the drawings. FIGS. 6 and 7 show cross-sectional views of a display pixel in the conventional liquid crystal display device using the FFS method. FIG. 6 shows the display pixel when no voltage is applied to a pixel electrode 12 that is to be described later, and FIG. 7 shows the display pixel when a voltage is applied to the pixel electrode 12.
A TFT (Thin Film Transistor) substrate 10 faces a light source BL as a first transparent substrate made of a glass substrate or the like, as shown in FIG. 6. A first polarizing plate 11 that linearly polarizes light from the light source BL is formed on a surface of the TFT substrate 10 facing the light source BL. The pixel electrode 12 to which the voltage is applied in response to a display signal is formed on another surface of the TFT substrate 10 that is not facing the light source BL. An insulating film 43 is formed on the pixel electrode 12 to cover it. Common electrodes 14 extending parallel to each other at predetermined intervals are formed on the insulating film 43. A first alignment film 15 covering the common electrodes 14 is formed on the insulating film 43.
A color filter substrate (hereafter referred to as CF substrate) 20 faces the common electrodes 14 as a second transparent substrate made of a glass substrate or the like. A color filter 21 and a second alignment film 22 are formed on a surface of the CF substrate 20 facing the common electrodes 14. A second polarizing plate 23 is formed on another surface of the CF substrate 20 that is not facing the common electrodes 14. The first and second polarizing plates 11 and 23 are disposed so that their polarization axes are perpendicular to each other. A liquid crystal layer 30 is sealed between the TFT substrate 10 and the CF substrate 20.
In the liquid crystal display device described above, an average direction of alignment (hereafter simply referred to as alignment direction) of liquid crystal molecules 31 in the liquid crystal layer 30 in a state where no voltage is applied to the pixel electrode 12 is perpendicular to a polarization axis of the first polarizing plate 11 in a plane parallel to the TFT substrate 10. The linearly polarized light outgoing from the first polarizing plate 11 does not go through the second polarizing plate 23 because its polarization axis is perpendicular to the polarization axis of the second polarizing plate 23. That is, black is displayed.
On the other hand, when the voltage is applied to the pixel electrode 12 to induce an electric field between the pixel electrode 12 and the common electrodes 14, the liquid crystal molecules 31 are rotated along lines of electric force of the electric field in a plane parallel to the TFT substrate 10, as shown in FIG. 7. In this case, the linearly polarized light outgoing from the first polarizing plate 11 becomes elliptically polarized light after traveling through the liquid crystal layer 30. That is, there is a component of linearly polarized light that goes through the second polarizing plate 23. In this case, white is displayed.
The alignment direction of the liquid crystal molecules 31 is tilted to a tangential direction of the lines of electric force of the electric field in a plane perpendicular to the TFT substrate 10 in the vicinity of the insulating film 43 between the common electrodes 14. That is, the alignment directions of the liquid crystal molecules 31 tilt symmetrically with respect to a center line between a neighboring pair of the common electrodes 14. Dependence on the viewing angle is cancelled out by the symmetrical tilt of the alignment directions, making it possible to achieve the wide viewing angle.
Technologies mentioned above are disclosed in Japanese Patent Application Publication No. 2002-296611.
Although the liquid crystal display device described above contributes to wider viewing angle in displaying white when the voltage is applied to the pixel electrode 12 to induce the electric field between the pixel electrode 12 and the common electrodes 14, it has not been considered sufficient. This invention is directed to improving the wide viewing angle characters of the conventional display device in which the alignment direction of the liquid crystal molecules is controlled by the electric field between the electrodes on the same substrate.